Fault code P1857 – a step-by-step guide to diagnosing and fixing it

Error code P1857 is displayed and the check engine light is on - what does this mean?

The OBD system detects an error in the transmission control unit, specifically in the pressure control of the automatic transmission. This DTC P1857 affects the transmission control unit and the hydraulic components of the automatic transmission. This error code can have different specific meanings for different manufacturers, but generally refers to problems with the transmission pressure control.

What causes can this error code have?

• Defective transmission control unit (TCM)
• Damaged pressure control valve in the valve body
• Worn transmission oil pump
• Blocked hydraulic channels in the transmission
• Defective pressure sensors in the transmission system
• Damaged wiring to the transmission control unit
• Loose or corroded plug connections
• Insufficient transmission oil level or contaminated ATF
• Mechanical wear of the transmission components
• Faulty solenoid valves in the valve body

What symptoms occur in parallel with error code P1857?

• Shifting problems: Jerky or delayed gear changes
• Emergency running program: The transmission only shifts in certain gears
• Loss of power: Reduced engine power and poor acceleration
• Unusual noises: Rattling or buzzing from the transmission area
• Overheating: Increased transmission temperature and corresponding warnings
• Check engine light: Permanently lit or flashing warning light
• Standstill: In severe cases, complete failure of the transmission

How can the problem of error code P1857 be rectified?

• The repair requires a systematic approach, as several components may be affected.
• Change the transmission oil: Complete replacement of the ATF oil including filter to eliminate hydraulic problems caused by contaminated oil
• Replace transmission control unit: Replace the defective TCM with a new or reconditioned control unit with subsequent programming
• Repair or replace valve body: Disassemble the transmission and replace the valve body including pressure control valves and solenoid valves
• Check wiring and plug connections: Complete check of all electrical connections to the transmission control unit and repair damaged wiring
• Replace pressure sensors: Replacement of defective pressure sensors in the transmission system after prior diagnosis
• Replace transmission oil pump: In the event of mechanical defects in the oil pump, complete replacement of the hydraulic pump unit
• Complete transmission overhaul: In the event of severe wear, complete disassembly, cleaning and replacement of all wear parts